Chapter 16 Modeling the solar interior The vibrating sun Neutrinos Solar atmosphere: –Photosphere –Chromosphere –Corona Sunspots Solar magnetic fields Active Sun

Questions about thermonuclear fusion Where does it take place? How does the energy produced by fusion make it to the surface of the Sun? Fusion requires very high pressures and temperatures, so it can’t be taking place everywhere in the sun. We can use the laws of physics to create models of the Sun.

Ingredients for solar model: 1.Hydrostatic equilibrium Sun is not dramatically expanding or collapsing 2.Thermal equilibrium Sun is not getting significantly hotter or cooler 3.Energy transport Rate of energy output = energy creation rate

Hydrostatic equilibrium At equilibrium the slab will not move up or down. Pressure below slab is higher than pressure above slab. This means pressure increases with increasing depth. –Pressure at certain depth is the same at all places in Sun.

Hydrostatic equilibrium

Thermal equilibrium Sun is so hot it is completely gaseous. Gases compress and become more dense when pressure increases. Compression of gas tends to heat it. This means that the temperature increases as you move toward the center of the Sun. –The temperature at any given depth is the same at all locations of the Sun at that depth.

Energy transport Convection Circulation between hot and cold (or hot and less hot) regions. Hot gas rises toward surface while cooler gas sinks back to center. Serves to transport energy created in center of Sun outward. Radiative diffusion Movement of photons outward from center of Sun. Photons are absorbed and re-emitted by atoms and electrons inside Sun. Eventually, these photons reach Sun’s surface and escape into space.

Solar models Astrophysicists express ideas of hydrostatic and thermal equilibrium as well as energy transport as a set of equations. Data for specific star is input and a computer solves equations and calculates conditions layer by layer inward toward the center of the star. Here are some results:

Solar models

Sun’s internal structure Within 0.71R energy transport is due to radiation. –Radiative zone Outside of 0.71R H atoms form and absorb photons easily. –Convective zone Center of Sun is dense so photons travel slowly, requiring 170,000 years to reach surface of Sun.

Solar vibrations How can we check if solar models are accurate? Helioseismology uses vibrations (detected by Doppler shift measurements) to study Sun. Periods ranging from minutes tell us about: –Amount of He in core and convective zone. –Thickness of transition region between zones.

Neutrinos Neutrinos are neutral particles that interact very weakly with matter. Each second are created in the Sun, of which pass through each square meter of the Earth. Observing these neutrinos allows for direct study of the Sun’s core.

Solar neutrino problem Detection of solar neutrinos would provide direct evidence of thermonuclear fusion. Early attempts to study neutrinos detected about 1 every 3 days but should have detected 3x more. Models wrong? Obvious changes didn’t work… Answer is neutrino oscillation.

Solar atmosphere

Solar atmosphere: photosphere Why does the Sun have such a sharp, well defined surface? Photosphere is only 400 km thick and very opaque. Opacity due to presence of negative hydrogen ions which absorb light very efficiently. Only 0.01% of the density of Earth’s atmosphere.

Granulation Surface of Sun appears blotchy. What causes this? –Granules equal in size to Texas and Oklahoma. Granulation caused by convection. Form, disappear and reform in a few minutes. –Watch video.

Solar atmosphere: chromosphere Very thin - about times the density of Earth’s atmosphere. Spectrum dominated by emission lines. –Red color due to H  light. Temperature increases with increasing height. –25,000 K at top of chromosphere. –Why? We’ll see soon. Spicules are jets of rising gas. –Lasts 15 minutes and rises at ~45,000 mph.

Solar atmosphere: corona Outermost layer called corona. Very hot, but not very bright since it is so tenuous. Emission lines first thought to be new elements. –Turned out in one case to be iron atoms with only 13 of 26 electrons. –T = 2x10 6 K for this to happen.

Temperature in upper atmosphere

Solar wind In general gravity keeps most of gas from escaping. Corona is so hot that that 10 9 kg escapes each second. –Called solar wind –Consists of electrons and H and He nuclei –Much less than 1% of Sun’s mass will ever be ejected. Coronal holes are sites with high outflow so gas is not as luminous.

Sunspots

Sunspot cycle

Magnetic fields in the Sun What causes sunspot cycle? The Sun’s magnetic field. First step towards answer in 1908 when George Hale discovered sunspots associated with intense magnetic fields. Hale observed “Zeeman splitting” of spectral lines which is caused by presence of strong magnetic field.

Charged particles in magnetic fields Due to high temperature, many atoms ionized. Electrically charged particles are deflected in a circular path by magnetic fields. Magnetic forces act on hot particles and deflect them away. –Result is localized region where gas is cool, or a sunspot.

Magnetic-dynamo model The Sun undergoes differential rotation which causes the magnetic field in the photosphere to be “wrapped” around the Sun. Field lines become twisted and “kinks” erupt through Solar surface. –Sunspots appear where the magnetic field protrudes through the photosphere.

Differential rotation (discovered via helioseismology)

Magnetic reconnection Gas of charged particles called plasma. Plasma tends to follow magnetic field lines forming large coronal loops. If two arches get too close they can rearrange and release massive amounts of energy. –Called magnetic reconnection –This explains the high temperatures in outer layers of the Sun.

Magnetic reconnection

Features on the active Sun Watch video of solar flares.

Coronal mass ejections Solar flares carry as much energy as megaton nuclear weapons. Coronal mass ejections are much larger. –1012 kg ejected into space at 100s km/s –Seem to be related to large scale alterations in Sun’s magnetic field. –CMEs can disrupt satellite communications if directed at Earth. –Astronomers still trying to understand CMEs.